Polyurethanes are polymers which are prepared by reacting a di- or polyisocyanate with a polyol. Polyurethanes are used in many different products and applications, including as insulation panels, high performance adhesives, high-resilience foam seating, seals and gaskets, wheels and tyres, synthetic fibres, and the like.
The polyols used to make polyurethanes are polymers which have multiple reactive sites (e.g. multiple hydroxyl functional groups). The polyols which are most commonly used are based on polyethers or polyesters.
One method for making polyether polyols in industry is by reacting an epoxide with a double metal cyanide (DMC) catalyst in the presence of a starter compound.
The nature and properties of the polyols have a great impact on the nature and the properties of the resultant polyurethanes. It is desirable to include polycarbonate linkages in the backbone of polyether polyols, as carbonate linkages in the polyol may improve the properties of the resultant polyurethane, for example, the presence of carbonate linkages may improve the UV stability, hydrolytic stability, chemical resistance and/or mechanical strength of the resulting polyurethane. The presence of carbonate linkages also increases the viscosity of the resulting polyol, which can limit use in some applications. It is therefore important to be able to control the ratio of ether linkages to carbonate linkages in polyols to tailor properties for widespread application. It is also important to be able to control the molecular weight and polydispersity of the polyol, as these properties impact usefulness and ease of processing of the resultant polyols.
DMC catalysts for use in the preparation of polyethers were first disclosed in U.S. Pat. No. 3,427,256 by The General Tyre and Rubber Company. It was subsequently found that carrying out this reaction in the presence of a starter compound yielded a polyether polyol.
DMC catalysts are also capable of preparing polyether polyols which contain carbonate linkages in the polymer backbone (hereinafter referred to as polycarbonate ether polyols). To prepare these types of polymers, the reaction is typically carried out at high pressures of carbon dioxide. It has generally been found that, for DMC catalysts, in order to obtain appreciable incorporation of carbon dioxide, the reaction must be carried out at pressures of 40 bar or above. This is undesirable as industrial equipment for preparing polyols are typically limited to pressures of up to 10 bar. For example, in US 2013/0072602, the examples set out the polymerisation of propylene oxide in the presence of a starter compound, and an additive at 50 bar CO2. The resulting polycarbonate ether polyols incorporate between 17.8 and 24.1 wt % CO2. Similar results can be seen in US 2013/0190462.
In WO 2015/022290, the examples show that when the polymerisation of propylene oxide is carried out in the presence of a DMC catalyst and a starter compound in the range of 15-25 bar CO2, the resulting polyols incorporated between 10.0 and 15.4 wt % CO2.
It is therefore desirable to be able to prepare polycarbonate ether polyols under pressures used in industrial polyether polyol equipment. It is also desirable to obtain appreciable incorporation of carbon dioxide (e.g. ≥20 wt % carbon dioxide, which requires a proportion of carbonate linkages of ˜0.5 in the polymer backbone, depending on the nature of the starter used) under low pressures.
WO 2010/028362 discloses a method for making polycarbonate polyols by copolymerising carbon dioxide and an epoxide in the presence of a chain transfer agent and a catalyst having a permanent ligand set which complexes a single metal atom. The polyols prepared in the examples have a proportion of carbonate linkages ≥0.95 in the polymer backbone. These systems are designed to prepare polycarbonates having little or no ether linkages in the polymer backbones. Furthermore, each of the examples is carried out at high pressures of 300 psig (about 20 bar) carbon dioxide.
WO 2013/034750 discloses a method for preparing polycarbonate polyols using a catalyst of formula (I):

The polyols prepared in the examples have ≥95% carbonate linkages, and generally ≥99% carbonate linkages in the polymer backbone.
WO 2012/121508 relates to a process for preparing polycarbonate ethers, which are ultimately intended for use as resins and soft plastics. This document is not concerned with preparing polyols. The process disclosed in WO 2012/121508 requires the copolymerisation of an epoxide and carbon dioxide in the presence of a DMC catalyst and a metal salen catalyst having the following formula:

The examples are each carried out at 16 bar CO2 or above. The resulting polycarbonate ethers contain varying amounts of ether and carbonate linkages. However, said polymers have a high molecular weight, have high polydispersity indices (that is, PDIs of 3.8 and above) and are not terminated by hydroxyl groups. These polymers cannot therefore be used to make polyurethanes.
Gao et al, Journal of Polymer Science Part A: Polymer Chemistry, 2012, 50, 5177-5184, describes a method for preparing low molecular weight polycarbonate ether polyol using a DMC catalyst and a di-carboxylic acid starter. The proportion of carbonate linkages can be increased up to 0.75 in the resultant polyols by decreasing the temperature (50° C.) and increasing the pressure (40 bar), when using a dicarboxylic acid starter which is apparently crucial to the ability to prepare polyols with high proportions of carbonate linkages. These conditions are unfavourable for economic industrial application. Gao et al suggests that dual catalysts systems for preparing polycarbonate ether polyols are unfavourable.
With previously reported catalyst systems, even at the widest range of temperature and pressures that have been deployed, it has reportedly not been possible to prepare polyols with proportions of carbonate linkages between 0.75 and 0.9.